Starting blocks have long been used by runners to position each foot at a chosen position at the starting line of a running track. That is, an individual runner has worked out over an extended period of training and experience, just exactly what position each of his feet should be at in order to make the best possible take off. Such a staring position may indeed determine the winner in a close race and accordingly the runner wants to have the blocks always resettable at the precise positions which he has found best, and always to have each block always firmly anchored without the possibility of wobble or slip in response to the pressure of his foot against it. Indeed to insure this, experienced runners may even carry their own (custom fashioned) starting block assembly with them from one track to another. Also, since such assemblies are continuously or practically always used in an immediate environment of either outdoor dirt or indoor dust and grit, it is important that the movable element or block setting-mechanism be both capable of secure anchorage at any chosen position along the bar and also that any slideway or track be kept reasonably free of interfering particles which may cause it to bind or jam. The realization of such objects is obtained in a notably effective manner by the present invention.